1. Field of the Invention
The present invention relates to a hydrodynamic type bearing unit. This bearing unit is particularly suited to supporting a spindle of a spindle motor for use in information equipment including magnetic disk drives such as an HDD and an FDD, optical disk drives such as a CD-ROM and a DVD-ROM, and magneto-optical disks such as an MD and an MO, or a spindle of a polygon scanner motor for use in laser beam printers (LBPs).
2. Description of the Related Art
A spindle motor in various information equipment mentioned above requires high rotational accuracy, along with higher speeds, lower costs, lower noises, and the like. Among the components that determine these performance requirements is a bearing for supporting the spindle of that motor. For this type of bearing, a hydrodynamic type bearing having excellent properties as to the above-mentioned performance requirements has been recently studied for its use, or put into practical use.
For example, some hard disk drives (HDD), a kind of information equipment, include a bearing unit in which, as schematically shown in FIG. 5, both a radial bearing part 10xe2x80x2 for supporting a spindle member 2xe2x80x2 in the radial direction and a thrust bearing part 11xe2x80x2 for supporting the spindle member 2xe2x80x2 in the thrust direction are constituted by hydrodynamic type bearings. In this shown example, the spindle member 2xe2x80x2 consists of a spindle 2axe2x80x2 and a thrust disc 2bxe2x80x2 fixed to an end of the spindle 2axe2x80x2. The rotation of the spindle member 2xe2x80x2 creates other hydrodynamic pressures of oil, air, or the like in a bearing clearance Crxe2x80x2 of the radial bearing part 10xe2x80x2 and in bearing clearances CS1xe2x80x2 and CS2xe2x80x2 of the thrust bearing part 11xe2x80x2, whereby the spindle member 2xe2x80x2 is rotatably supported in both the radial and thrust directions without contact.
Recently, in contemplation of on-notebook-PC mounting and the like, there has been a growing demand that the above-described bearing unit 1xe2x80x2 have more compact configuration, in particular a reduced axial dimension (lower profile). As a measure to be taken therefor, the radial bearing part 10xe2x80x2 may be effectively reduced in axial length Lxe2x80x2. The reduction of the axial length Lxe2x80x2, however, is accompanied by a decrease of the radial bearing part 10xe2x80x2 in bearing span (distance between axially-spaced hydrodynamic type bearings), producing a problem of lower bearing capacity against a moment load.
In particular, the thrust disc 2bxe2x80x2 and the spindle 2axe2x80x2 conventionally involve forming errors in the perpendicularity therebetween and the like. To prevent the thrust disc 2bxe2x80x2 and the thrust bearing surfaces 11axe2x80x2, 11bxe2x80x2 from contacting each other because of these errors, the thrust bearing clearances CS1xe2x80x2 and CS2xe2x80x2 are formed fairly large. When the spindle member 2xe2x80x2 here is tilted by a moment load (as shown by double-dashed chain lines in FIG. 5), however, most of the moment load is supported by the radial bearing part 10xe2x80x2 (the top end thereof, in particular) while the thrust bearing part 11xe2x80x2 exerts little moment supporting power. This decreases the moment load capacity of the entire bearing unit.
Moreover, when the bearing unit 1xe2x80x2 is made thinner as described above, the diameter Dxe2x80x2 of the thrust disc 2bxe2x80x2 becomes greater in relation to the axial length Lxe2x80x2 of the radial bearing part 10xe2x80x2. The moment load capacity of the entire bearing unit thus depends largely on the moment load capacity of the thrust bearing part 11xe2x80x2. On the contrary, the conventional thrust bearing part 11xe2x80x2 cannot provide a sufficient moment supporting power, and this has been a major obstacle to increase moment load capacity.
In view of the foregoing, an object of the present invention is to enhance the moment supporting power, in particular, of the thrust bearing part, and improve the moment load capacity of the entire hydrodynamic type bearing unit.
To achieve the foregoing object, a hydrodynamic type bearing unit according to the present invention comprises: a spindle member composed of a spindle and a flange portion; a radial bearing part for supporting the spindle member in a radial direction without contact by means of a hydrodynamic pressure occurring in a radial bearing clearance between the outer periphery of the spindle and a radial bearing surface opposed thereto; and a thrust bearing part for supporting the spindle member in a thrust direction without contact by means of hydrodynamic pressures occurring in thrust bearing clearances between the end faces of the flange portion and thrust bearing surfaces opposed thereto, wherein a relation between the radial bearing clearance and the thrust bearing clearances are set so that a moment load acting on the spindle member is supported by both of the radial bearing part and the thrust bearing part. Here, the spindle member tilted comes into contact at the outer periphery of its spindle with both axial ends of the radial bearing surface opposed thereto. In the meantime, the rims of both end faces of its flange portion also come into contact with the respective thrust bearing surfaces opposed thereto.
When the radial bearing clearance xcex4R and the thrust bearing clearance xcex4A satisfy the following expression:
(Dxcex4R/L)+xcex5xe2x89xa7xcex4Axe2x89xa7Dxcex4R/L
(where D is the diameter of the flange portion, L is the axial length L of the radial bearing part, and xcex5 is a machining error), both the radial bearing part and the thrust bearing part can secure a practically sufficient moment supporting power.
Here, the machining error xcex5 may be 4 xcexcm. The diameter D of the flange portion may be equal to or smaller than 10 mm.
In any of the configurations described above, the diameter of the flange portion is preferably greater than the axial length of the radial bearing part.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.